When chemists respond creatively to the abstract world of design, and then execute their designs through a practice, which has become known as synthesis, then chemistry becomes an extremely powerful artistic medium in which to forge new materials with potentially awesome functions. Over more than a couple of centuries now, chemists have come to appreciate the role of atoms that constitute the different elements to not only form molecules, but also to make molecules. While nature bears adequate witness to the former, the chemist is the maker of molecules with no holds barred. Thus, chemists have explored the interactions between the atoms and molecules at the covalent, coordinative (dative) and noncovalent levels of bonding. During the past half century, a growing appreciation of the importance of the noncovalent bond has propelled chemistry beyond the molecule to what Jean-Marie Lehn (Lehn, J. M., Supramolecular Chemistry: Concepts and Perspectives, Wiley-VCH, 1995, 271 pp) refers to as supramolecular chemistry. In more recent times, the concepts of molecular recognition and self-assembly (Whitesides, G. M. et al., Science 2002, 295, 2418-2421) have been employed in syntheses that are template-directed (Griffiths, K. E. et al., Pure Appl. Chem. 2008, 80, 485-506) in order to create efficiently an additional chemical bond, namely the mechanical bond (Stoddart, J. F. et al., Tetrahedron 2008, 64, 8231-8263; Stoddart, J. F., Chem. Soc. Rev. 2009, 38, 1802-1820). By introducing the mechanical bond into chemistry, chemists have managed to integrate weak noncovalent with strong covalent bonding to create within molecules a unique range of intriguing properties with the potential for far-reaching applications in the rapidly expanding arena of molecular nanotechnology (Kay, E. R. et al., Angew. Chem. Int. Ed. 2007, 46, 72-191).
Today, there is a need for chemists to reach out beyond the molecule in another way that is every bit as robust as building mechanical bonds into molecules; it is to create extended networks of atoms by designing and constructing new crystalline solids from molecular building blocks, such that the molecule is the “crystal” and the crystal is the “molecule”. Endowed potentially with many of the properties (e.g., binding, reactivity, catalysis, etc.) that small and not so small molecules enjoy, these so-called (Yaghi, O. M. et al., J. Am. Chem. Soc. 1995, 117, 10401-10402) metal-organic frameworks (MOFs) have given rise to a completely new field of materials science that has been christened (Yaghi, O. M. et al., “Reticular Synthesis and the Design of New Materials,” Nature 2003, 423, 705-714) “reticular chemistry” by its leading proponent, Omar Yaghi. During the past decade, the field of reticular chemistry has developed a pace in the research laboratories, not only of Yaghi (Yaghi, O. M. et al., Nature 1995, 378, 703-706; Eddaoudi, M. et al., Science 2002, 295, 469-472; Rosi, N. L. et al., Science 2003, 300, 1127-1129; Chen, B. et al., Angew. Chem. Int. Ed. 2005, 44, 4745-4749; Sudik, A. C. et al., J. Am. Chem. Soc. 2005, 127, 7110-7118; Rowsell, J. et al., Angew. Chem. Int. Ed. 2005, 44, 4670-4679; Millward, A. R. et al., J. Am. Chem. Soc. 2005, 127, 17998-17999; Cote, A. P. et al., Science 2005, 310, 1166-1170; Rowsell, J. et al., Science 2005, 309, 1350-1354; Wong-Foy, A. G. et al., J. Am. Chem. Soc. 2006, 128, 3494-3495; Kaye, S. S. et al., J. Am. Chem. Soc. 2007, 129, 14176-14177; Walton, K. S. et al., J. Am. Chem. Soc. 2008, 130, 406-407; El-Kaderi, H. M. et al., Science 2007, 316, 268-272; Banerjee, R. et al., Science 2008, 319, 939-943), but also of Férey (Livage, C. et al., Angew. Chem. Int. Ed. 2005, 44, 6488-6491; Horcajada, P. et al., Angew. Chem. Int. Ed. 2006, 45, 5974-5978; Latroche, M.; Surblé, S.; Serre, C.; Mellot-Draznieks, C.; Llewellyn, P. L. et al., Angew. Chem. Int. Ed. 2006, 45, 8227-8231; Loiseau, T. et al., J. Am. Chem. Soc. 2006, 128, 10223-10230; Férey, G. et al., Angew. Chem. Int. Ed. 2007, 46, 3259-3263), Kitagawa (Matsuda, R. et al., Nature 2005, 436, 238-241; Kubota, Y. et al., Angew. Chem. Int. Ed. 2005, 44, 920-923), Hupp (Farha, O. K. et al., J. Am. Chem. Soc. 2007, 129, 12680-12681; Lee, J. Y. et al., Chem. Soc. Rev. 2009, 38, 1450-1459; Gadzikwa, T. et al., Chem. Commun. 2009, 3720-3722; Mulfort, K. L. et al., Inorg. Chem. 2008, 47, 7936-7938) and many others (Han, S. S. et al., J. Am. Chem. Soc. 2007, 129, 8422-8423; Han, S. S. et al., Angew. Chem. Int. Ed. 2007, 46, 6289-6292; Mulfort, K. L. et al., J. Am. Chem. Soc. 2007, 129, 9604-9605; Ma, L. Q. et al., Chem. Soc. Rev. 2009, 38, 1248-1256; Chen, S. M. et al., Inorg. Chem. 2009, 48, 6356-6358; Burrows, A. D. et al., Chem. Commun. 2009, 4218-4220; Blomqvist, A. et al., Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 20173-20176; Tanabe, K. K. et al., J. Am. Chem. Soc. 2008, 130, 8508-8517; An, J. et al., J. Am. Chem. Soc. 2009, 131, 8401-8403; Dinca, M. et al., Angew. Chem. Int. Ed. 2008, 47, 6766-6779; Thallapally, P. K. et al., J. Am. Chem. Soc. 2008, 130, 16842-16843). The fundamentals of the science are taking shape very quickly and are being fueled by the enormous potential these highly porous reticular materials hold. Many, including Robert Service (Service, R. F. “Framework Materials Grab CO2 and Researchers' Attention,” Science 2008, 319, 893) at Science, believe that “MOFs and related compounds are one of the hottest playgrounds in chemistry” today.
The synthesis of complex and highly functional structures through the self-assembly of simple building blocks is commonly observed both in nature (Angelescu, D. G. et al., Soft Matter 2008, 4, 1981-1990; Koltover, I. Nat. Mater. 2004, 3, 584-586; Yeager, M. et al., A. Proc. Nat. Acad. Sci. 1998, 95, 7299-7304) and in the laboratory (Conn, M. et al., Chem. Rev. 1997, 97, 1647-1668; Kim, J. et al., Angew. Chem. Int. Ed. 2007, 46, 7393-73; Vriezema, D. M. et al., Chem. Rev. 2005, 104, 1445-1490). This “bottom-up” approach has also proved promising for the fabrication of nanoscale materials and devices (Bath, J. et al., Nat. Nanotechol. 2007, 2, 275-284; Ulijn, R. V. et al., Chem. Soc. Rev. 2008, 37, 664-675; Endo, M. et al., Chem. Eur. J. 2007, 13, 8660-8666; Wang, Q. et al., Chemistry & Biology 2002, 9, 813-819). In the last decade, this strategy has led to the discovery of MOFs (Eddaoudi, M. et al., Acc. Chem. Res. 2001, 34, 319-330; Li, H. et al., Nature 1999, 402, 276-279), which have demonstrated great promise as storage materials for gaseous small molecules in carbon capture and clean energy applications (Britt, D. et al., Proc. Nat. Acad. Sci. 2008, 105, 11623-11627; Rowsell, J. L. C. et al., Angew. Chem. Int. Ed. 2005, 44, 4670-4679). In addition, other molecules (e.g., drugs, small organic molecules) can be stored in the cavities of MOFs, opening the possibility for smart drug delivery devices or molecular sequestration (An, J. et al., J. Am. Chem. Soc. 2009, 131, 8376-8377; Horcajada, P. et al., J. Am. Chem. Soc. 2008, 130, 6774-6780; Sanchez, C. et al., J. Mater. Chem. 2005, 15, 3559-3592). The large void spaces in MOFs can also be utilized as a scaffold for the placement of molecular receptors yielding robust, nanoscale devices in the solid state.
In a specific example, sequestration of carbon dioxide from gaseous waste streams in the purification of petrol compounds has become a pressing issue for the scientific and global community in light of the predicted detrimental effects of anthropogenic CO2 production. Recently, several approaches toward this goal have emerged using metal organic frameworks (MOFs) derived from petrochemical sources (Rowse J. L. C. et al., Microporous Mesoporous Mater. 2004, 73, 3; Kitagawa, S. et al., Angew. Chem., Int. Ed. 2004, 43, 2334; Ferey, G. Chem. Soc. ReV, 2008, 37, 191; Li, J. R., et al., Chem. Soc. ReV. 2009, 38, 1477). Free hydroxyl and amine residues are known to react with carbon dioxide to form carbonic acids and carbamic acids respectively. These functionalities have been added to MOFs by rational design of struts (Caskey, S. R. et al., 3.1. Am. Chem. Soc. 2008, 130, 10870; Demessence, A. et al., J. Am. Cheat. Soc. 2009, 131, 8784; Ban, Y. S. et al., J. Mater. Chem. 2009, 19, 2131; Arstad, B. et al., Adsorption 2008, 14, 755; Banerjee, R. et al., J. Am. Chesil. Soc. 2009, 131, 3875; Vaidhyanathan, R. et al., Chem. Commun. 2009, 5230; Chen, S. M.; Zhang, J. et al., J. Am. Chem. Soc. 2009, 131, 16027; An, J. et al., J. Ant. Chem. Soc. 2009, 131, 8401. While these advances are noteworthy in their incremental storage capacity, they are generally synthesized from environmentally malevolent materials and solvents. Recent examples have been reported that use biological molecules, but these biomolecules do not comprise the fullness of the MOF. Further, once they have fully adsorbed the full content of their gaseous payload, they do not provide a mechanism by which to alert an end user that the material needs to be emptied or changed.
However the majority of MOF structures reported to date are composed of toxic heavy metals and struts derived from non-renewable petrochemical feedstocks and assembled in harmful organic solvents at high pressures and temperature. Therefore, the assembly of functional materials from simple components that are renewable and biocompatible is desirable in a wide variety of applications, from drug delivery devices (non-toxic) (Wang, X. et al., CA—Cancer J. Clin. 2008, 58, 97-110) to nanoscale device fabrication (Dankers, P. Y. W. et al., Bull. Chem. Soc. Jpn. 2007, 80, 2047-2073; Stefano, L. et al., Chem. Eur. J. 2009, 15, 7792-7806).